The invention relates to a device for measuring the attentuation of optical waves on optical transmission paths designed with lightwave guides, said device comprising an optical transmitter, exhibiting a light source and a connection for the path to be measured, and an optical receiver. In order to determine the transmission quality of a transmission path constructed from lightwave guides, the magnitude of the useful signal arriving at the end can be compared with the magnitude of the signal fed into the transmission path, and thus the attenuation of the path can be determined.
From U.S. Pat. No. 4,021,121 a device for measuring interference locations and/or the lengths of glass fibers is known, which comprises an impulse laser and a light-sensitive receiver. The light impulse emitted by the impulse laser penetrates a first focusing optic and subsequently impinges upon a beam divider, for example, upon a partially permeable reflector. The one light share coming from the beam divider is directed towards the glass fiber to be examined with the aid of a second focusing optic; the impulse runs through the glass fiber, is reflected at its end or at an interference location and subsequently reaches the beam divider again in order to strike now the light-sensitive receiver via an additional focusing optic. An additional method for locating a crack on at least one fiber of an optical cable is known from U.S. Pat. No. 4,012,149; this method is based upon a transit time measurement.
Now it is known that the result of attenuation measurements depends upon the mode spectrum excited by means of the irradiating light source. This can be attributed to the fact that not all modes in a lightwave guide propagate with the same efficiency and that therefore light shares, which propagate in unfavorable modes, are lost for the useful output of the communication transmission. In a lightwave guide of the core-jacket type, the main share of the light is propagated in the core. Light, which is conveyed in the core, exhibits various courses. Their wave shapes are called core modes. The light can be axially conveyed in the core material; however, due to the reflection at the border surfaces of the core material it can also propagate in a zig-zag course. Therefrom result various propagation times for the different modes due to the different path lengths. If the light propagating in a zig-zag course includes such a large angle with the border surface of the core that the border angle for total reflection is exceeded, then the light can also reach the jacket area and then proceeds only in the jacket area between the outer and the inner surface of the jacket material due to the reflections at the border surfaces of the jacket area. It thereby deals with jacket modes. Depending upon the course of the light it is also possible that the light is partially conveyed in the core and partially in the jacket of the lightwave guide. These modes are called leakage modes. Since such wave shapes are particularly strongly attenuated, they get lost after a certain course path.
As every lightwave guide practically exhibits some form of irregularities, for example, a not precisely straight course of the lightwave guide, all kinds of modes can be newly formed in the lightwave guide due to mode transformation, which modes subsequently disappear again due to the attenuation typical for them after a certain path length, or said modes are again transformed into different modes. Accordingly, on a lightwave guide path, at least at the beginning, a continuous transition from one wave mode into a different mode and a corresponding attenuation takes place until finally--after a sufficiently long path (approximately 500 to 1000 meters) an end condition of the mode distribution is reached, which is then retained in the further course of the path and which is determining for the attenuation of the transmitted light. Thus, it does matter in what modes the light, which is irradiated for the purpose of measuring in a lightwave guide, is propagated. A measurement with light of such a mode distribution, which does not agree with the mode distribution of the light conveyed in the lightwave guide in a practical operation, would lead to different and therefore to wrong measuring results. This phenomenon is also known from the hollow waveguide technique.